Questions to experts in Variable Amplitude Loading in Fatigue of metals

As a newcomer in this field, I try to pose some questions to the experts in Variable Amplitude Loading fatigue. The questions are probably trivial, but I don't find any reply in the literature, particularly on the arbitrary choice of frequency cutoff in the spectrum loading, and on the distinction between cracked and uncracked structures. See attach and please respond if you can, even directly on email at mciava@poliba.it